Some embodiments of the invention relate to an associated self-calibration method for a laser tracker.
In this case, a laser tracker of the type mentioned in the introduction comprises a base defining a vertical axis, a support and a beam directing unit for emission of measurement radiation and for reception of at least part of the measurement radiation reflected at a target. The beam directing unit is oriented in two axes (vertical axis and inclination axis or tilting axis), by means of motors. In this case, the support is pivotable in a motorized fashion about the vertical axis relative to the base, and the beam directing unit about a tilting axis relative to the support. A measuring axis is defined by an emission direction of the measurement radiation.
The beam directing unit is equipped with opto-electro-mechanical components and is mounted at one or two mounting locations on the support rotatably about the tilting axis by means of a shaft, said support likewise being equipped with opto-electro-mechanical components, if appropriate.
Laser trackers as coordinate measuring machines belong to a type of measuring machines which measure the coordinates of a (spatial) point by emitting a laser beam onto the point. The laser beam may impinge directly on the point or on a retroreflector (often a cube corner prism or “corner cube” or arrangement having three mirrors oriented perpendicularly to one another) which is in contact with the point. In the case of a retroreflector, the laser beam impinging thereon is reflected “on itself”, i.e. coaxially with respect to the emitted laser beam, if the latter impinges exactly on the center of the retroreflector. Otherwise, if the emitted laser beam impinges on the retroreflector outside the center thereof, the reflected laser beam has a parallel offset with respect to the emitted laser beam.
The machine typically determines the coordinates of the point by measuring the distance between the point and the measuring machine and two angles by means of angle encoders or angle sensors assigned to rotation axes of the laser tracker between a standard orientation of the laser beam with regard to its targeting direction with respect to the point to be measured. The distance is measured by a distance measuring device, such as, for example, an absolute distance measuring device and/or an interferometer. Exemplary systems for determining coordinates of a point are disclosed in U.S. Pat. No. 4,790,651 and U.S. Pat. No. 4,714,339.
Laser trackers are a special type of coordinate measuring machines used to track an, in particular moving, target point, in particular embodied as a retroreflector, by means of one or a plurality of, in particular focused, laser beams.
Reliable use of laser trackers that is reproducible in the measurement result necessitates the setting and application of calibration parameters. Calibration parameters are typically stored as numerical values in the form of software or firmware in a manner accessible to the laser tracker controller and, when applied to the raw measurement data of the laser tracker, serve to improve the measurement accuracy. Typically, the manufacturer of the laser tracker carries out so-called calibration measuring methods for determining the calibration parameters and stores the corresponding calibration parameters with the control software. On the machine side, certain tolerances regarding the extent to which current calibration parameters are allowed to deviate from previously stored calibration parameters are usually additionally defined with the control. In order to determine changes in the machine calibration, monitoring calibration measurements are typically carried out at specific intervals and/or when the laser tracker is switched on.
Changes in the required machine calibration are based in particular on thermal drift effects, but also on mechanical vibrations, for example.
EP 1 420 264 discloses a laser tracker and a measuring method implementable therewith with calibration devices and specifications. A measuring system is described which comprises a measuring machine having a laser tracker and an optoelectronic sensor in invariable positions relative to one another, a system computer and a separate auxiliary measuring instrument, i.e. which is to be arranged at a distance from the laser tracker, with a reflector and at least three light points. The laser tracker is calibrated by means of the method steps described below: the auxiliary measuring instrument is rigidly connected to an arrangement of auxiliary reflectors and moved about at least two rotation axes that differ from one another relative to the auxiliary measuring instrument. In at least two respective rotation positions about each of the at least two rotation axes, reflector and auxiliary reflectors are targeted by the laser tracker and the light points of impinging laser light are registered by the optoelectronic sensor. Positions and orientations of the reflector arrangement relative to the laser tracker are determined from the measurement data of the laser tracker and positions and orientations of the light point arrangement relative to the optoelectronic sensor are determined from the measurement data of the optoelectronic sensor and the at least two rotation axes relative to the reflector arrangement and to the light point arrangement are calculated therefrom. The calibration data are then calculated from the measurement data ascertained.
This system arrangement and the calibration method associated therewith do not correspond to the arrangement nor to the typically imposed specifications of a laser tracker according to the present invention and, in particular, nor do they correspond to present-day requirements made of such a measuring system.
Particularly disadvantageously, the auxiliary measuring instrument for the calibration is arranged outside the measuring machine or laser tracker, which does not satisfy requirements made of present-day laser trackers for the fullest possible compactly arranged integration or combination with the measuring machine, and a self-calibration method comprising automatically proceeding, machine-controlled method steps without the involvement of an operator cannot be gathered from EP 1 420 264.
US 2009/0109426 and WO 2005/026772 disclose a self-calibrating laser tracker comprising a laser for emitting a laser beam, a plane mirror and at least two integrated immobile, reflective devices and also a rotatable mirror and a position-sensitive detector. One of the at least two immobile, reflective devices is embodied as a corner cube retroreflector, and a second as a plane mirror. The corner cube retroreflector and the plane mirror can be fixed in position on a stationary part of the measuring system and are designed to reflect the laser beam according to a two-position measuring method, i.e. in a “front-side mode” and a “rear-side mode”. In this case, the “front-side mode” corresponds to the orientation of the laser tracker in accordance with a regular target tracking, and the “rear-side mode” corresponds to an opposite orientation of the laser tracker.
In accordance with the arrangements disclosed in US 2009/0109426 and WO 2005/026772, measurement values of temperature sensors arranged on the machine are used to ascertain a temperature dependence of the values to be determined for the calibration parameters.
The arrangements disclosed in US 2009/0109426 and WO 2005/026772 are disadvantageous, however, owing to the need to use a complexly producible individual retroreflector such as a cube corner retroreflector having precisely defined or produced reflective surfaces and very stringent requirements made of its exact positioning for the purposes of reliable self-calibration of the alignment of a laser tracker.